This invention relates to a method of prilling material and particularly to a method of operation which produces new and improved prilling results.
In prilling molten material the material is discharged into the top of a tower in the form of droplets and the droplets assume a spherical or spheroidal shape in falling through the tower and solidify during their descent. The prilled material thus formed is then gathered or removed from the bottom of the tower. It is conventional to provide a counter current of cooling air which flows upwardly through the tower to hasten the cooling of the droplets.
In conventional prilling methods of the prior art substantial quantities of the air passing upwardly through the tower are released to the atmosphere and such air usually contains finely divided material which produces air pollution.
Furthermore, in most, or at least in many, prilling operations a portion of the material accumulates against the interior walls of the prilling chamber and often builds up to substantial amounts so that ultimately chunks of the material fall to the bottom of the tower, causing damage to the structure.
It is well understood in the prilling art that the droplets discharged at the top of the tower must fall freely downwardly through the tower in assume the requisite spherical or spheroidal shape and solidify to form prills. Accordingly conventional devices at the top of the tower for discharging droplets of material are arranged so that the droplets will fall freely out of contact with the walls of the tower since contact with such walls would deform the droplets and not form proper prills.
However, it is found that in prilling operations relatively fine particles of material of a size not large enough to be called droplets are inevitably discharged with the droplets, such fine particles being too small to form the desired prills. These fine particles are of such small size that they tend to drift about in the tower instead of falling directly as do the droplets of proper size to form prills. This drifting about is accentuated by the counter current of cooling air mentioned above which flows upwardly through the tower to hasten the cooling of the droplets. It is these fine unprilled particles which form the undesired accretions of material against the interior walls of the prilling chamber.